Dimethylnaphthalene (hereinafter, referred to as “DMN”) has ten isomers. In general, organic compounds are purified by operations such as distillation, crystallization, and adsorption and combinations thereof. Unfortunately, these dimethylnaphthalene isomers, which have significantly small differences in melting point and boiling point, cannot be readily purified by distillation or crystallization. Dimethylnaphthalene mixtures have been separated by conventional methods such as crystallization and adsorption.
For example, the following methods are known: Using ethyl alcohol as a solvent, 2,7-dimethylnaphthalene (hereinafter, referred to as “2,7-DMN”) is selectively precipitated from a dimethylnaphthalene isomer mixture, and is separated by filtration while the DMNs having low melting points are present in a liquid state (refer to Patent Document 1); a DMN isomer mixture containing 2,7-DMN is treated with methanol and 2,7-DMN is isolated by crystallization (refer to Patent Document 2); high-purity 2,7-DMN is isolated from a stock oil containing DMN isomers by high-pressure crystallization (refer to Patent Document 3); and 2,7-DMN and 2,6-dimethylnaphthalene are separated from a DMN isomer mixture by high-selectivity adsorption with zeolite having ten-membered rings as an adsorbent (see Patent Document 4).
Unfortunately, no industrial method with low costs and a high yield through a simple procedure is established to isolate high-purity 2,7-dimethylnaphthalene from a dimethylnaphthalene mixture and particularly a mixture of 2,7-dimethylnaphthalene and 1,7-dimethylnaphthalene (hereinafter, referred to as “1,7-DMN”).
Resin made of naphthalene dimethyl diester, as a polyester raw material, that is prepared by oxidation/esterification of dimethylnaphthalene containing these isomers exhibit poor physical and mechanical characteristics, such as low thermal resistance, low mechanical strength, and low dimensional stability. Thus, this cannot be used as a raw material for polyesters. Commercially available methods of separation of high-purity dimethylnaphthalene have been studied over a period of years.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 9-124520    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 48-22448    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 4-120027    [Patent Document 4] Japanese Unexamined Patent Application Publication No. 1-224336